As you can see below, the FIRST Lego League is still anticipating the release of NXT in August. They are making the announcement for the 2007 Challenge in Mid-Sept, so students will have about 2-4 weeks to experiment with the new system from the time they receive it.
I'll put in a plug for volunteering for the FLL. Currently the Atlanta, Georgia (USA) volunteer coordinator is looking for adults to volunteer to help with the 3 day event. If you're interested, I can email you the contact information for faster response (the online app procedure takes a little time for you to hear back)
Current Robot Sets, FIRST DVD's, and Coaches' Handbooks begin to ship
New Release Robot Sets and Field Setup Kits begin to ship
International Challenge Announced